The invention relates to high-pressure discharge lamps which include structure for preventing failure of the outer envelope in the event of explosive rupture of the discharge vessel.
More specifically, high pressure discharge lamps such as metal halide lamps typically include an outer envelope sealed in a vacuum-tight manner and having an axis and an end portion fused with a mount and arranged in a lamp cap provided with contacts. A gas-filled discharge vessel having vacuum-tight seals is axially mounted in the outer envelope. An electric element, such as a pair of discharge electrodes, are axially arranged in the discharge vessel and connected to a first and a second current lead-through conductor emanating from the discharge vessel through a first and a second seal, respectively, near and remote from the mount, respectively. A glass cylindrical sleeve axially is arranged in the outer envelope to surround the lamp vessel and having a first end near the mount and a second end, which sleeve is closed at least at one end. A first and a second current supply conductor extending from the contacts at the lamp cap through the mount to the first and the second current lead-through conductor to energize the discharge vessel to emit light. Frame structure within the lamp vessel and the glass cylindrical sleeve positioned in the outer envelope.
Such an electric lamp is known from GB 495,978. In order to ensure that lamps having a very high operating pressure during operation can be used safely, it is necessary for the lamp vessel to be enveloped within the outer envelope. It should be prevented that upon explosion of the lamp vessel, fragments thereof leave the outer envelope. An explosion can occur both when the electric element is a pair of electrodes and when this element is a filament.
In the known lamp, the mount is provided with a frame, in which the lamp vessel is mounted. A set of wire springs keeps the glass cylindrical sleeve transversely positioned in the outer envelope. A current supply conductor cooperates with a projection on the outer envelope so as to keep both the lamp vessel transversely fixed and the glass cylindrical sleeve transversely and axially fixed.
The construction of the known lamp is not sufficient to ensure that the lamp can be safely used. In fact, the glass cylindrical sleeve is open at one end. Moreover, the construction is complicated. Another disadvantage is that the lamp has a comparatively large diameter, which is disadvantageous with the use in an optical system, in which event the lamp has to be mounted, for example, on the focal line of a reflector. A wide outer envelope then imposes limitations on the shape of the reflector.